Embodiments of the present invention are directed to power supply control circuits and power supply systems and their applications. More particularly, embodiments of the present invention provide methods for systems for controlling a switched mode power supply for providing constant output current in LED light systems.
A DC-DC converter receives a rectified DC voltage and delivers a regulated DC output. DC-DC converters are widely used in white light-emitting diode (LED) drivers or flash LED drivers. Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency, and larger output power capability. On the other hand, they also have the disadvantages of greater noise, especially Electromagnetic Interference at the power transistor's switching frequency or its harmonics.
Conventional power supplies of buck-boost topology use current control mode (CCM) or voltage control mode (VCM) loop control that needs internal or outside compensation, which can often cause circuit instability. Compared with the ordinary structure of CCM or VCM switch controller, the architecture described in FIG. 1 tends to be more stable. Such a controller is extensively used in home-lighting, auto-motor, and backlight instruments. In LED lighting systems, the LEDs are often connected in series in the inductor loop.
FIG. 1 is a schematic diagram of an LED lighting system 100 driven by a conventional switching mode power supply. As shown in FIG. 1, lighting system 100 includes serially connected multiple LEDs 104 coupled with a load capacitor 111. The LEDs are driven by a power supply 120, which includes a sense resistor 101, an inductor 102, and a Schottky diode 103. Power supply 120 also includes a controller 130, which includes a transconductance amplifier 105, a Dim linear amplifier 106, a current adder 107, a resistor 108, a comparator 109, and a power MOSFET 110. As shown, transconductance amplifier 105 receives inputs from both ends of sense resistor 101, and power transistor 110 is connected to a node between inductor 102 and Schottky diode 103.
As shown in FIG. 1, power supply 120 receives a rectified DC input voltage Vin. During the charging period, the current from Vin flows through resistor 101 and inductor 102 through power transistor 110 to ground. In this period, energy is stored in inductor 102. The voltage across resistor 101 is sensed by transconductance amplifier 105, which produces an output current I1. Current I1 is fed to resistor 108 through current adder 107, and the resulting voltage is compared with an internal voltage reference Vref. When an internal turn-on reference voltage is reached, the output of comparator 109 drives power MOSFET 110 to switch off through a drive block (not shown). In the discharging period, the energy stored in inductor 102 discharges through diode 103 which, along with capacitor 111, provide a current Iout to LEDs 104. When the sense voltage becomes lower than an internal off reference voltage and detected by comparator 109, power transistor 110 is turned on again, and the charging period is repeated. Controller 130 is capable of boosting input voltage Vin to a higher regulated output voltage Vout.
Even though conventional LED lighting systems, such as system 100 of FIG. 1, can be found in many application, they suffer from many limitations. These limitations include, for example, instability in light output, which may result in flickers.
Therefore, it is desirable to have improved methods and devices for controlling the output current in a power supply in LED lighting and other applications.